To better understand the mechanisms involved in mammary gland cancer induction by the heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and the effect of dietary fat, studies are underway to assess the affect of PhIP and dietary fat on the mammary gland epithelial cell proliferation, apoptosis, and glandular development in an in vivo rat model. Female Sprague-Dawley rats were administered a carcinogenic dose regimen of PhIP and placed on either a high fat or low fat diet as described in project Z01CP05748-3. At various times after carcinogen administration, mammary glands from rats were examined for premalignant histological alterations. Epithelial cell proliferation and apoptosis were examined by immunohistochemical methods. Mammary whole mounts are currently being set-up to monitor the development and branching of the gland from carcinogen-treated and control animals on high and low fat diets. Data suggest that carcinogen treatment produces a time-dependent atrophy of the mammary gland. The rate of apoptosis in the glands was different between control rats and PhIP-treated animals on high and low fat diets. Proliferation also appeared to be influenced by carcinogen treatment. Further studies are underway to define the molecular alterations associated with the carcinogen plus dietary fat-induced premalignant changes in the rat mammary gland.